1. Technical Field of the Invention
The present invention generally relates to wireless communication networks. In particular, it relates to a method and system for mobile-station-uplink-transmit bit-rate adaptation to extend coverage area of a cell and to reduce a required uplink carrier signal/interference signal (C/I) level.
2. Description of Background and Related Art
Network operators estimate that in the near future there will be more than 1 billion mobile phone subscribers worldwide. By then, the number of wireless phones may equal or even exceed the number of wired phones. It is clear that the next significant development in wireless communications involves the standardization of third-generation wireless services. Third-generation wireless services will enable subscribers to, for example, make video calls to friends and colleagues from the mobile station, while simultaneously accessing a remote database from that same mobile station, or receiving e-mails and phone calls.
One possible platform or standard for third-generation wireless services is known as Wideband Code Division Multiple Access (WCDMA). The WCDMA standard supports both packet-switched and circuit switched communications such as Internet browsing and traditional landline telephone services, respectively. The WCDMA Standard has been submitted to the International Telecommunication Union (ITU). The ITU is the body that selects the platforms or standards that are to support the third-generation services to be used in what is known as a universal mobile telecommunications system (UMTS).
In WCDMA networks, a closed loop power control (PC) operates on both an uplink and a downlink of a network and is responsible for maintaining required link quality. The PC decreases or increases transmit power to keep a carrier-signal-to-interference-signal level (C/I) measured at a receiving side as close as possible to a required C/I level so that acceptable link quality is maintained. The required C/I level is set by the network in order to attempt to maintain acceptable link quality, which is defmed in terms of frame error rate (FER) for speech and block error rate (BLER) for packet data transmissions. The higher the C/I, the lower the FER and BLER.
When a mobile station operating within a cell served by a base station approaches a border of the cell, the uplink power control attempts to maintain the uplink quality by increasing the transmit power of the mobile station. The increase in transmit power by the mobile station continues until the transmit power of the mobile station reaches a maximal value beyond which the mobile station cannot further increase its transmit power. Once the mobile station is transmitting at maximal power, it can no longer respond to further degradation of the uplink C/I by increasing its transmit power more. Therefore, if the maximal mobile station transmit power has already been reached, the mobile station is unable to further increase its power to adapt to increased FER and/or BLER resulting from the degraded C/I.
Uplink quality degradation is especially strong if a cell breathing effect of the network is stressed by a large number of high-bit-rate-data-capable mobile stations being served by the network. Cell breathing refers to coverage fluctuations of the cell due to increased traffic. High connection reliability is often needed for packet-data services requested by the high-bit-rate-data-capable mobile stations; therefore, applications that use high-bit-rate packet-data services can usually tolerate only very low error rates. An Automatic Repeat reQuest (ARQ) protocol can handle errors by retransmitting packets received in error. However, because the C/I degrades as the high-bit-rate-capable mobile stations using the packet-data services move closer to the cell border, the ARQ orders more and more retransmissions of packets received in error. As a result of the retransmissions, these high bit-rate data capable mobile stations do not get adequate throughput. Therefore, these mobile stations typically achieve an acceptable uplink quality only when they are close enough to the base station that the required C/I level is present.
It is well known that decreasing an uplink mobile station transmit bit-rate by increasing a spreading factor decreases the C/I level required to ensure a desired FER or BLER. The lower required C/I level at a lower bit rate can be fulfilled with less mobile station transmit power, with the result being that the uplink quality and coverage of the cell improve. Decreasing the uplink transmit bit rate is particularly well-suited for packet-data transmission schemes in which a set of transport format combinations is assigned to each of the mobile stations, as in WCDMA. Each transport format combination set delineates transmit bit-rates available to certain mobile stations. Thus, under WCDMA, when a given mobile station decreases its transmit bit rate, it is adapting to a transport format combination that corresponds to a lower bit rate within the transport format combination set (TFCS) assigned to the given mobile station.
A mobile-station-managed-bit-rate-reduction-based approach to improving the coverage area of a cell is described in Minimum Requirement for UE radio link adaptation function, TSG-RAN Working Group 4 (Radio) meeting #7, TSGW#7(99)492, Source: Nokia (hereinafter xe2x80x9cthe Nokia paperxe2x80x9d). The Nokia paper describes a mobile-station-managed bit-rate decrease based on a power-alarm threshold and averaging time requirements. The approach described in the Nokia paper requires that when maximum transmit power of a mobile station has been reached and the closed-loop power control can no longer be maintained, the mobile station (also referred to as user equipment, or UE) lowers its transmit bit-rate within a specified maximal time period to the next lower bit rate within its assigned transport format combination set. The Nokia paper""s approach has been approved as a minimum requirement for user equipment radio link adaptation function for a Third Generation Partnership Project (3GPP) technical specification entitled Requirements for Support of Radio Resource Management, (FDD) TDS 25.133 v. 2.2.0. Network-managed-bit-rate-reduction-based approaches are also possible in which, for example, a radio network controller (RNC) orders a bit-rate reduction responsive to receipt of a power-alarm trigger from the mobile station.
A mobile-station-handled approach would most likely be better suited for solving local coverage-area problems than a network-handled approach because it would avoid delays in information exchange between the network (e.g., the RNC) and the mobile stations. This is because a network-handled approach is by its very nature more centralized that a mobile-station handled approach and therefore would almost certainly react more slowly to C/I degradation due to, for example, extra delays occasioned by information being exchanged between the network and the mobile station.
While the Nokia paper teaches a mobile-station-handled approach and therefore arguably avoids the information-exchange-delay drawback of network-handled approaches, the Nokia paper does not provide for mobile-station transmit bit-rate increases responsive to subsequent improvements in C/I. This failure is due to concerns about network instability that can result from rapid oscillations between higher and lower uplink-transmit bit-rates, which are sometimes referred to as ping-pong effects. If, in order to avoid ping-pong effects, a bit-rate-increase functionality is not included, unnecessarily-low throughput occurs after the C/I improves. This is especially true in best-effort services in which the highest possible bit-rate has been requested by mobile stations.
In order to extend a coverage area of the cell and decrease a required C/I level, a method and system are needed that permit a closed loop power control (PC) to quickly respond to C/I degradation and to maintain link quality as the distance of the mobile station from the base station increases. This need is especially strongly felt when the network is heavily loaded with the high-bit-rate-data-capable mobile stations that stress the breathing effect of the cell. In the event that the uplink-transmit bit rate of the mobile stations has been decreased and the C/I subsequently improves, it would be desirable for the uplink-transmit bit rate of the mobile stations to be increased in response thereto. However, the variable uplink-transmit bit-rate functionality should not result in ping-pong effects.
These and other drawbacks are overcome by the present invention. A method of transmitting at various bit rates to improve coverage of a cell comprises the steps of transmitting by a mobile station at a first bit rate, transmitting by the mobile station at a second bit rate responsive to a determination that a transmit power of the mobile station exceeds an alarm-power threshold, and transmitting by the mobile station at a third bit rate. The transmission by the mobile station at the third bit rate is responsive to a determination that the transmit power of the mobile station has not exceeded a lower-power threshold for a predetermined time period. The lower-power threshold can be set so that the step of transmitting by the mobile station at the third bit rate does not cause the transmit power of the mobile station to exceed the alarm-power threshold.
A bit rate adaptation system in accordance with the present invention comprises a mobile station and a network serving the mobile station. The mobile station transmits at a first bit rate until a transmit power of the mobile station exceeds an alarm-power threshold, transmitting at a second bit rate responsive to the transmit power exceeding the alarm-power threshold, and transmitting at a third bit rate responsive to the transmit power being less than a lower-power threshold for a predetermined time period. The lower-power threshold can be set by the mobile station at the third bit rate so that a bit-rate increase does not cause the transmit power of the mobile station to exceed the alarm-power threshold.
A method of transmitting at various bit rates to improve coverage area of a cell comprises the steps of transmitting by a mobile station at a first bit rate, transmitting by the mobile station of the second bit rate, estimating a transmit power corresponding to transmission by the mobile station at a third bit rate, and transmitting by the mobile station at the third bit rate. The step of transmitting by the mobile station at the second bit rate is responsive to a determination that the transmit power of the mobile station has exceeded an alarm-power threshold for a predetermined time period. The step of transmitting by the mobile station at the third bit rate is responsive to a determination that the estimated transmit power would not exceed the alarm-power threshold.
A bit rate adaptation system in accordance with the present invention comprises a mobile station and a network serving the mobile station. The mobile station transmits at a first bit rate until a transmit power of the mobile station exceeds an alarm-power threshold for a predetermined time period, transmits at a second bit rate responsive to the transmit power exceeding the alarm-power threshold for the predetermined time period, and transmits at a third bit rate responsive to a determination by the mobile station that the transmit power corresponding to the third bit rate would not exceed the alarm-power threshold.